FIGS. 14 and 15 are cross-sectional views of two different prior art monolithic convolvers in which reference numeral 1 refers to a high-concentration semiconductor substrate, 2 to an insulator, 3 to a piezoelectric film, 4 and 4' to inputs, 5 and 5' to comb-shaped electrodes, 6 to an output, 7 to a back electrode, 8 to a gate electrode, and 9 to a semiconductive epitaxial film.
Arrangements of FIGS. 14 and 15 are different in semiconductor structure. In FIG. 14, a bulk semiconductor substrate is used as the semiconductor, whereas the arrangement of FIG. 15 uses a substrate made by growing a semiconductive epitaxial film on a high-concentrated semiconductor substrate (a substrate having a high impurity density). Various properties of these arrangements are reported in the following publications.
Document [1], B. T. Khuri--Yakub and G. S. Kino, "A Detailed Theory of the Monolithic Zinc Oxide on Silicon Convolver", IEEE Trans. Sonics Ultrason., vol. SU - 24, No. 1, January 1977, pp. 34-43 PA0 Document [2], J. K. Elliott, et al. "A Wideband SAW convolver utilizing Sezawa waves in the metal--ZnO--SiO.sub.2 --Si configuration", Appl. Phys. Lett. 32, May 1978, pp. 515-516 PA0 Document [3], J. E. Bowers, et al. "Monolithic Sezawa wave Storage Correlators and Convolvers", IEEE Proc. Ultrasonics Symposium, 1980, pp. 118-123 PA0 Document [4], S. Minagawa, et al. "Efficient ZnO--SiO.sub.2 --Si Sezawa wave convolver", IEEE Trans. Sonics Ultrason., vol. SU - 32, No. 5, September 1985, pp. 670-674 PA0 (i) F.sub.T increases with decrease of R.sub.B. PA0 (ii) F.sub.T increases with decrease of the propagation loss.
Documents [1], [2]and [3]report properties of the convolver of FIG. 14, and Document [4] reports properties of the convolver of FIG. 15.
As will be understood from comparison of these reports, the arrangement of FIG. 15 tends to exhibit a higher value than that of FIG. 14. In this respect, the use of a substrate including an epitaxial film as shown in FIG. 15 is more advantageous as a convolver. However, no theoretical or experimental detailed discussion has been made heretofore. Therefore, no definite data exists heretofore about the best condition of such an epitaxial layer, and no convolver having the best arrangement is not provided heretofore.